Cathode materials for secondary (rechargeable) lithium batteries

ABSTRACT

The invention relates to materials for use as electrodes in an alkali-ion secondary (rechargeable) battery, particularly a lithium-ion battery. The invention provides transition-metal compounds having the ordered-olivine or the rhombohedral NASICON structure and the polyanion (PO 4 ) 3  as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.

This is a continuation application of application Ser. No. 11/179,617,filed Jul. 13, 2005 now abandoned, which is a continuation ofapplication Ser. No. 10/902,142, filed Jul. 30, 2004 (now abandoned),which is a continuation of application Ser. No. 10/307,346, filed Dec.2, 2002 (now abandoned), which is a continuation of application Ser. No.08/998,264, filed Dec. 24, 1997 (now U.S. Pat. No. 6,514,640, issuedFeb. 4, 2003), which is a continuation-in-part of application Ser. No.08/840,523 (now U.S. Pat. No. 5,910,382, issued Jun. 8, 1999), filedApr. 21, 1997. This application claims priority through theabove-identified applications to provisional patent Application No.60/032,346, filed Dec. 4, 1996, and provisional patent Application No.60/016,060, filed Apr. 23, 1996. The entire text of each of theabove-referenced disclosures is specifically incorporated by referenceherein without disclaimer. The Robert A. Welch Foundation, Houston,Tex., supported research related to the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to secondary (rechargeable) alkali-ionbatteries. More specifically, the invention relates to materials for useas electrodes for an alkali-ion battery. The invention providestransition-metal compounds having the ordered olivine or therhombohedral NASICON structure and containing the polyanion (PO₄)³ as atleast one constituent for use as an electrode material for alkali-ionrechargeable batteries.

2. Description of the Related Art

Present-day lithium batteries use a solid reductant as the anode and asolid oxidant as the cathode.

On discharge, the metallic anode supplies Li⁺ ions to the Li⁺-ionelectrolyte and electrons to the external circuit. The cathode istypically an electronically conducting host into which Li⁺ ions areinserted reversibly from the electrolyte as a guest species andcharge-compensated by electrons from the external circuit. The chemicalreactions at the anode and cathode of a lithium secondary battery mustbe reversible. On charge, removal of electrons from the cathode by anexternal field releases Li⁺ ions back to the electrolyte to restore theparent host structure, and the addition of electrons to the anode by theexternal field attracts charge-compensating Li⁺ ions back into the anodeto restore it to its original composition.

Present-day rechargeable lithium-ion batteries use a coke material intowhich lithium is inserted reversibly as the anode and a layered orframework transition-metal oxide is used as the cathode host material(Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/orNi are expensive and may degrade due to the incorporation of unwantedspecies from the electrolyte. Oxides such as Li_(1±x)[Mn₂]O₄, which hasthe [M₂]O₄ spinel framework, provide strong bonding in three dimensionsand an interconnected interstitial space for lithium insertion. However,the small size of the O²⁻ ion restricts the free volume available to theLi⁺ ions, which limits the power capability of the electrodes. Althoughsubstitution of a larger S²⁻ ion for the O²⁻ ion increases the freevolume available to the Li⁺ ions, it also reduces the output voltage ofan elementary cell.

A host material that will provide a larger free volume for Li⁺-ionmotion in the interstitial space would allow realization of a higherlithium-ion conductivity σ_(Li), and hence higher power densities. Anoxide is needed for output voltage, and hence higher energy density. Aninexpensive, non-polluting transition-metal atom within the hoststructure would make the battery environmentally benign.

SUMMARY OF THE INVENTION

The present invention meets these goals more adequately than previouslyknown secondary battery cathode materials by providing oxides containinglarger tetrahedral oxide polyanions forming 3D framework host structureswith octahedral-site transition-metal oxidant cations, such as iron,that are environmentally benign.

The present invention provides electrode material for a rechargeableelectrochemical cell comprising an anode, a cathode and an electrolyte.The cell may additionally include an electrode separator. As usedherein, “electrochemical cell” refers not only to the building block, orinternal portion, of a battery but is also meant to refer to a batteryin general. Although either the cathode or the anode may comprise thematerial of the invention, the material will preferably be useful in thecathode.

Generally, in one aspect, the invention provides an ordered olivinecompound having the general formula LiMPO₄, where M is at least onefirst-row transition-metal cation. The alkali ion Li⁺ may beinserted/extracted reversibly from/to the electrolyte of the batteryto/from the interstitial space of the host MPO₄ framework of theordered-olivine structure as the transition-metal M cation (orcombination of cations) is reduced/oxidized by charge-compensatingelectrons supplied/removed by the external circuit of the battery in,for a cathode material, a discharge/charge cycle. In particular, M willpreferably be Mn, Fe, Co, Ti, Ni or a combination thereof. Examples ofcombinations of the transition-metals for use as the substituent Minclude, but are not limited to, Fe_(1-x)Mn_(x), and Fe_(1-x)Ti_(x),where 0≦x≦1.

Preferred formulas for the ordered olivine electrode compounds of theinvention include, but are not limited to LiFePO₄, LiMnPO₄, LiCoPO₄,LiNiPO₄, and mixed transition-metal compounds such asLi_(1-2x)Fe_(1-x)Ti_(x)PO₄ or LiFe_(1-x)Mn_(x)PO₄, where 0≦x≦1. However,it will be understood by one of skill in the art that other compoundshaving the general formula LiMPO₄ and an ordered olivine structure areincluded within the scope of the invention.

The electrode materials of the general formula LiMPO₄ described hereintypically have an ordered olivine structure having a plurality of planesdefined by zigzag chains and linear chains, where the M atoms occupy thezigzag chains of octahedra and the Li atoms occupy the linear chains ofalternate planes of octahedral sites.

In another aspect, the invention provides electrode materials for arechargeable electrochemical cell comprising an anode, a cathode and anelectrolyte, with or without an electrode separator, where the electrodematerials comprise a rhombohedral NASICON material having the formulaY_(x)M₂ (PO₄)₃, where 0≦x≦5. Preferably, the compounds of the inventionwill be useful as the cathode of a rechargeable electrochemical cell.The alkali ion Y may be inserted from the electrolyte of the battery tothe interstitial space of the rhombohedral M₂(XO₄)₃ NASICON hostframework as the transition-metal M cation (or combination of cations)is reduced by charge-compensating electrons supplied by the externalcircuit of the battery during discharge with the reverse processoccurring during charge of the battery. While it is contemplated thatthe materials of the invention may consist of either a singlerhombohedral phase or two phases, e.g. orthorhombic and monoclinic, thematerials are preferably single-phase rhombohedral NASICON compounds.Generally, M will be at least one first-row transition-metal cation andY will be Li or Na. In preferred compounds, M will be Fe, V, Mn, or Tiand Y will be Li.

Redox energies of the host M cations can be varied by a suitable choiceof the XO₄ polyanion, where X is taken from Si, P, As, or S and thestructure may contain a combination of such polyanions. Tuning of theredox energies allows optimization of the battery voltage with respectto the electrolyte used in the battery. The invention replaces the oxideion O²⁻ of conventional cathode materials by a polyanion (XO₄)^(m−) totake advantage of (1) the larger size of the polyanion, which canenlarge the free volume of the host interstitial space available to thealkali ions, and (2) the covalent X—O bonding, which stabilizes theredox energies of the M cations with M-O—X bonding so as to createacceptable open-circuit voltages V_(o), with environmentally benignFe³⁺/Fe²⁺ and/or Ti⁴⁺/Ti³⁺ or V⁴⁺/V³⁺ redox couples.

Preferred formulas for the rhombohedral NASICON electrode compounds ofthe invention include, but are not limited to those having the formulaLi_(3+x)Fe₂(PO₄)₃, Li_(2+x)FeTi(PO₄)₃, Li_(x)TiNb(PO₄)₃, andLi_(1+x)FeNb(PO₄)₃, where 0<x<2. It will be understood by one of skillin the art that Na may be substituted for Li in any of the abovecompounds to provide cathode materials for a Na ion rechargeablebattery. For example, one may employ Na_(3+x)Fe₂(PO₄)₃,Na_(2+x)FeTi(PO₄)₃, Na_(x)TiNb(PO₄)₃ or Na_(1+x)FeNb(PO₄)₃, where 0<x<2,in a Na ion rechargeable battery. In this aspect, Na⁺ is the working ionand the anode and electrolyte comprise a Na compound.

Compounds of the invention having the rhombohedral NASICON structureform a framework of MO₆ octahedra sharing all of their corners with XO₄tetrahedra (X=Si, P, As, or S), the XO₄ tetrahedra sharing all of theircorners with octahedra. Pairs of MO₆ octahedra have faces bridged bythree XO₄ tetrahedra to form “lantern” units aligned parallel to thehexagonal c-axis (the rhombohedral [111] direction), each of these XO₄tetrahedra bridging to two different “lantern” units. The Li⁺ or Na⁺ions occupy the interstitial space within the M₂(XO₄)₃ framework.Generally, Y_(x)M₂(XO₄)₃ compounds with the rhombohedral NASICONframework may be prepared by solid-state reaction of stoichiometricproportions of the Y, M, and XO₄ groups for the desired valence of the Mcation. Where Y is Li, the compounds may be prepared indirectly from theNa analog by ion exchange of Li⁺ for Na⁺ ions in a molten LiNO₃ bath at300° C. For example, rhombohedral LiTi₂(PO₄)₃ may be prepared fromintimate mixtures of Li₂CO₃ or LiOH.H₂O, TiO₂, and NH₄H₂PO₄.H₂O calcinedin air at 200° C. to eliminate H₂O and CO₂ followed by heating in airfor 24 hours near 850° C. and a further heating for 24 hours near 950°C. However, preparation of Li₃Fe₂(PO₄)₃ by a similar solid-statereaction gives the undesired monoclinic framework. To obtain therhombohedral form, it is necessary to prepare rhombohedral Na₃Fe₂(PO₄)₃by solid-state reaction of NaCO₃, Fe{CH₂COOH}₂ and NH₄H₂PO₄.H₂O, forexample. The rhombohedral form of Li₃Fe₂(PO₄)₃ is then obtained at 300°C. by ion exchange of Li⁺ for Na⁺ in a bath of molten LiNO₃. It will beunderstood by one of skill in the art that the rhombohedral Na compoundswill be useful as cathode materials in rechargeable Na ion batteries.

In another aspect of the invention, the rhombohedral NASICON electrodecompounds may have the general formula Y_(x)M₂(PO₄)_(y)(XO₄)_(3-y),where 0<y≦3, M is a transition-metal atom, Y is Li or Na, and X═Si, As,or S and acts as a counter cation in the rhombohedral NASICON frameworkstructure. In this aspect, the compound comprises a phosphate anion asat least part of an electrode material. In preferred embodiments, thecompounds are used in the cathode of a rechargeable battery. Preferredcompounds having this general formula include, but are not limited toLi_(1+x)Fe₂(SO₄)₂(PO₄), where 0≦x≦1.

The rhombohedral NASICON compounds described above may typically beprepared by preparing an aqueous solution comprising a lithium compound,an iron compound, a phosphate compound and a sulfate compound,evaporating the solution to obtain dry material and heating the drymaterial to about 500° C. Preferably, the aqueous starting solutioncomprises FeCl₃, (NH₄)₂SO₄, and LiH₂PO₄.

In a further embodiment, the invention provides electrode materials fora rechargeable electrochemical cell comprising an anode, a cathode andan electrolyte, with or without an electrode separator, where theelectrode materials have a rhombohedral NASICON structure with thegeneral formula A_(3-x)V₂(PO₄)₃. In these compounds, A may be Li, Na ora combination thereof and 0≦x≦2. In preferred embodiments, the compoundsare a single-phase rhombohedral NASICON material. Preferred formulas forthe rhombohedral NASICON electrode compounds having the general formulaA_(3-x)V₂(PO₄)₃ include, but are not limited to those having the formulaLi_(2−x)NaV₂(PO₄)₃, where 0≦x≦2.

The rhombohedral NASICON materials of the general formulaA_(3-x)V₂(PO₄)₃ may generally be prepared by the process outlined inFIG. 9. Alternatively, Li₂NaV₂(PO₄)₃ may be prepared by a directsolid-state reaction from LiCO₃, NaCO₃, NH₄H₂PO₄.H₂O and V₂O₃.

In a further aspect, the invention provides a secondary (rechargeable)battery where an electrochemical cell comprises two electrodes and anelectrolyte, with or without an electrode separator. The electrodes aregenerally referred to as the anode and the cathode. The secondarybatteries of the invention generally comprise as electrode material, andpreferably as cathode material, the compounds described above. Moreparticularly, the batteries of the invention have a cathode comprisingthe ordered olivine compounds described above or the rhombohedralNASICON compounds described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to demonstrate further certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1. FIG. 1 shows a typical polarization curve for the batteryvoltage V vs. the I delivered across a load. The voltage drop(V_(oc)−V)≡η(I) of a typical curve is a measure of the batteryresistance Rb(I). The interfacial voltage drops saturate in region (i).The slope of the curve in region (ii) is dV/dI≈R_(e1)+R_(c)(A)+R_(c)(C),the sums of the electrolyte resistance R_(e1) and the current-collectorresistances at the anode and cathode. Region (iii) is diffusion-limited.At the higher currents I, normal processes do not bring ions to orremove them from the electrode/electrolyte interfaces rapidly enough tosustain an equilibrium reaction.

FIGS. 2A, 2B and 2C. FIG. 2A shows discharge/charge curves at 0.05mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1−x)FePO₄ as cathode andlithium as anode. A plateau at 3.4V corresponds to the Fe³⁺/Fe²⁺ redoxcouple relative to the lithium anode. FIG. 2B shows discharge/chargecurves at 0.05 mA·cm⁻² (1.13 mA·g⁻¹) for the olivineLi_(1−x)Fe_(0.5)Mn_(0.5)PO₄ as cathode relative to a lithium anode. Aplateau at 3.4V corresponds to the Fe³⁺/Fe²⁺ redox couple relative tothe lithium anode. A plateau at 4.1 V corresponds to the Mn³⁺/Mn²⁺couple. FIG. 2C shows discharge/charge curves vs. lithium at 0.05mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1−x)FePO₄.

FIG. 3. FIG. 3 shows discharge/charge curves of anFePO₄/LiClO₄+PC+DME/Li coin cell at 185 mA·g⁻¹ for FePO₄ prepared bychemical extraction of Li (delithiation) from LiFePO₄.

FIG. 4. FIG. 4 shows a schematic representation of the motion ofLiFePO₄/FePO₄ interface on lithium insertion in to a particle of FePO₄.

FIGS. 5A and 5B. FIG. 5A shows the rhombohedral R3c (NASICON) frameworkstructure of Li₃Fe₂(PO₄)₃ prepared by ion exchange from Na₃Fe₂(PO₄)₃;FIG. 5B shows the monoclinic P2₁/n framework structure of Li₃Fe₂(PO₄)₃prepared by solid-state reaction. The large, open three-dimensionalframework of FeO₆ octahedra and PO₄ tetrahedra allows an easy diffusionof the lithium ions.

FIGS. 6A and 6B. FIG. 6A shows discharge/charge curves vs. lithium at0.1 mA·cm⁻² for rhombohedral Li_(3+x)Fe₂(PO₄)₃ where 0<x<2. The shape ofthe curve for lithium insertion into rhombohedral Li_(3+x)Fe₂(PO₄)₃ issurprisingly different from that for the monoclinic form. However, theaverage V_(oc) at 2.8 V remains the same. The Li⁺-ion distribution inthe interstitial space appears to vary continuously with x with a highdegree of disorder. FIG. 6B shows discharge/charge curves vs. lithium at0.1 mA·cm⁻² for monoclinic Li_(3+x)Fe₂(PO₄)₃ where 0<x<2.

FIGS. 7A and 7B. FIG. 7A shows discharge curves vs. a lithium anode atcurrent densities of 0.05-0.5 mA·cm⁻² for rhombohedralLi_(3+x)Fe₂(PO₄)₃. A reversible capacity loss on increasing the currentdensity from 0.05 to 0.5 mA·cm⁻² is shown. This loss is much reducedcompared to what is encountered with the monoclinic system. FIG. 7Bshows discharge curves at current densities of 0.05-0.5 mA·cm⁻² formonoclinic Li_(3+x)Fe₂(PO₄)₃.

FIG. 8. FIG. 8 shows discharge/charge curves at 0.05 mA·cm⁻² (0.95mA·g⁻¹) for the rhombohedral Li₂NaV₂(PO₄)₃. Rhombohedral Li₂NaV₂(PO₄)₃reversibly intercalates 1.5 Li per formula unit for a discharge capacityof 100 mAh·g⁻¹ with average closed-circuit voltage of 3.8 V vs. alithium anode.

FIG. 9. FIG. 9 illustrates the solid-state synthesis of Li₂NaV₂(PO₄)₃having the rhombohedral NASICON framework.

FIG. 10. FIG. 10 shows discharge/charge curves vs. lithium at 0.1mA·cm⁻² for rhombohedral Li_(1+x)Fe₂(PO₄)(SO₄)₂ where 0≦x≦2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Present-day secondary (rechargeable) lithium batteries use a solidreductant as the anode, or negative electrode, and a solid oxidant asthe cathode, or positive electrode. It is important that the chemicalreactions at the anode and cathode of a lithium secondary battery bereversible. On discharge, the metallic anode supplies Li⁺ ions to theLi⁺-ion electrolyte and electrons to the external circuit. The cathodeis a host compound into/from which the working Li⁺ ion of theelectrolyte can be inserted/extracted reversibly as a guest species overa large solid-solubility range (Goodenough 1994). When the Li⁺ ions areinserted as a guest species into the cathode, they arecharge-compensated by electrons from the external circuit. On charge,the removal of electrons from the cathode by an external field releasesLi⁺ ions back to the electrolyte to restore the parent host structure.The resultant addition of electrons to the anode by the external fieldattracts charge-compensating Li⁺ ions back into the anode to restore itto its original composition.

The present invention provides new materials for use as cathodes inlithium secondary (rechargeable) batteries. It will be understood thatthe anode for use with the cathode material of the invention may be anylithium anode material, such as a reductant host for lithium orelemental lithium itself. Preferably, the anode material will be areductant host for lithium. Where both the anode and cathode are hostsfor the reversible insertion or removal of the working ion into/from theelectrolyte, the electrochemical cell is commonly called a“rocking-chair” cell. An implicit additional requirement of a secondarybattery is maintenance not only of the electrode/electrolyte interfaces,but also of electrical contact between host particles, throughoutrepeated discharge/recharge cycles.

Since the volumes of the electrode particles change as a result of thetransfer of atoms from one to another electrode in a reaction, thisrequirement normally excludes the use of a crystalline or glassyelectrolyte with a solid electrode. A non-aqueous liquid or polymerelectrolyte having a large energy-gap window between its highestoccupied molecular orbital (HOMO) and its lowest unoccupied molecularorbital (LUMO) is used with secondary lithium batteries in order torealize higher voltages. For example, practical quantities of very ioniclithium salts such as LiClO₄, LiBF₄ and LiPF₆ can be dissolved inempirically optimized mixtures of propylene carbonate (PC), ethylenecarbonate (EC), or dimethyl carbonate (DMC) to provide acceptableelectrolytes for use with the cathodes of the invention. It will berecognized by those of skill in the art that the (ClO₄)⁻ anion isexplosive and not typically suitable for commercial applications.

General Design Considerations

The power output P of a battery is the product of the electric current Idelivered by the battery and the voltage V across the negative andpositive posts (equation 1).P−IV  (1)The voltage V is reduced from its open-circuit value V_(oc) (I=0) by thevoltage drop IR_(b) due to the internal resistance R_(b) of the battery(equation 2).V=V _(oc) −IR _(b)  (2)The open-circuit value of the voltage is governed by equation 3.V _(oc)=(μ_(A)−μ_(C))/(−nF)<5V  (3)In equation 3, n is the number of electronic charges carried by theworking ion and F is Faraday's constant. The magnitude of theopen-circuit voltage is constrained to V_(oc)<5V not only by theattainable difference μ_(A)−μ_(C) of the electrochemical potentials ofthe anode reductant and the cathode oxidant, but also by the energy gapE_(g) between the HOMO (highest occupied molecular orbital) and the LUMO(lowest unoccupied molecular orbital) of a liquid electrolyte or by theenergy gap E_(g) between the top of the valence band and the bottom ofthe conduction band of a solid electrolyte.

The chemical potential μ_(A), which is the Fermi energy ε_(F) of ametalic-reductant anode or the HOMO of a gaseous or liquid reductant,must lie below the LUMO of a liquid electrolyte or the conduction bandof a solid electrolyte to achieve thermodynamic stability againstreduction of the electrolyte by the reductant. Similarly, the chemicalpotential μ_(A)−μ_(C), which is the LUMO of a gaseous or liquid oxidantor the Fermi energy of a metallic-oxidant cathode, must lie above theHOMO of a liquid electrolyte or the valence band of a solid electrolyteto achieve thermodynamic stability against oxidation of the electrolyteby the oxidant. Thermodynamic stability thus introduces the constraintμ_(A)−μ_(C) ≦E _(g)  (4)as well as the need to match the “window” E_(g) of the electrolyte tothe energies μ_(A) and μ_(C) of the reactants to maximize V_(oc). Itfollows from equations 1 and 2 that realization of a high maximum powerP_(max) (equation 5) requires, in addition to as high a V_(oc) aspossible, a low internal battery resistance Rb (see equation 6).P _(max) =I _(max) V _(max)  (5)R _(b) =R _(el)(A)+R _(in)(C)+R _(c)(A)+R _(c)(C)  (6)The electrolyte resistance R_(el) to the ionic current is proportionalto the ratio of the effective thickness L to the geometrical area A ofthe interelectrode space that is filled with an electrolyte of ionicconductivity σ_(i) (equation 7).R _(el)=(L/σ _(i) A)  (7)

Since ions move diffusively, σ_(i) (see equation 8) increases withtemperature. A σ_(i)≦0.1 Scm⁻¹ (the maximum σ_(i) represents theroom-temperature protonic conductivity σ_(H) in a strong acid) at anoperating temperature T_(op) dictates the use of a membrane separator oflarge geometrical area A and small thickness L.σ_(Li)=(B/T)exp(−E _(v) /kT)  (8)

The resistance to transport of the working ion across theelectrolyte-electrode interfaces is proportional to the ratio of thegeometrical and interfacial areas at each electrode:R _(in) ˜A/A _(in)  (9)where the chemical reaction of the cell involves ionic transport acrossan interface, equation 9 dictates construction of a porous,small-particle electrode. Achievement and retention of a high electrodecapacity, i.e., utilization of a high fraction of the electrode materialin the reversible reaction, requires the achievement and retention ofgood electronic contact between particles as well as a largeparticle-electrolyte interface area over many discharge/charge cycles.If the reversible reaction involves a first-order phase change, theparticles may fracture or lose contact with one another on cycling tobreak a continuous electronic pathway to the current collector.

Loss of interparticle electrical contact results in an irreversible lossof capacity. There may also be a reversible capacity fade. Where thereis a two-phase process (or even a sharp guest-species gradient at adiffusion front) without fracture of the particles, the area of theinterface (or diffusion front) decreases as the second phase penetratesthe electrode particle. At a critical interface area, diffusion acrossthe interface may not be fast enough to sustain the current I, so notall of the particle is accessible. The volume of inaccessible electrodeincreases with I, which leads to a diffusion-limited reversible capacityfade that increases with I. This problem becomes more important at lowerionic conductivity σ_(Li).

The battery voltage V vs. the current I delivered across a load iscalled the polarization curve. The voltage drop (V_(oc)−V)≡η(I) of atypical curve, FIG. 1, is a measure of the battery resistance (seeequation 10).R _(b)(I)=η(I)/I  (10)On charging, η(I)=(V_(app)−V_(oc)) is referred to as an overvoltage. Theinterfacial voltage drops saturate in region (i) of FIG. 1; therefore inregion (ii) the slope of the curve isdV/dI=R _(el) +R _(c)(A)+R _(c)(C)  (11)Region (iii) is diffusion-limited; at the higher currents I, normalprocesses do not bring ions to or remove them from theelectrode/electrolyte interfaces rapidly enough to sustain anequilibrium reaction.

The battery voltage V vs. the state of charge, or the time during whicha constant current I has been delivered, is called a discharge curve.

Cathode Materials

The cathode, or positive electrode, material of the present invention,for use in a secondary lithium battery, consists of a host structureinto which lithium can be inserted reversibly. The maximum power output,P_(max) (see equation 5) that can be achieved by a cell depends on theopen-circuit voltage V_(oc)=ΔE/e and the overvoltage η(I) at the currentI_(max) of maximum powerV _(max) =V _(oc)−η(I)  (12)ΔE is the energy difference between the work function of the anode (orthe HOMO of the reductant) and that of the cathode (or the LUMO of theoxidant). In order to obtain a high V_(oc), it is necessary to use acathode that is an oxide or a halide. It is preferable that the cathodebe an oxide in order to achieve a large V_(oc) and good electronicconductivity. To minimize η(I_(max)), the electrodes must be goodelectronic as well as ionic conductors and they must offer a lowresistance to mass transfer across the electrode/electrolyte interface.To obtain a high I_(max), it is necessary to have a largeelectrode/electrolyte surface area. In addition, where there is atwo-phase interface within the electrode particle, the rate of masstransfer across this interface must remain large enough to sustain thecurrent. This constraint tends to limit the electrode capacity more asthe current increases.

Oxide host structures with close-packed oxygen arrays may be layered, asin Li_(1−x)CoO₂ (Mizushima, et al. 1980), or strongly bonded in threedimensions (3D) as in the manganese spinels Li_(1−x)[Mn₂]O₄ (Thackeray1995; Thackeray et al. 1983; Thackeray et al. 1984; Guyomard andTarascon 1992; and Masquelier et al. 1996). Li intercalation into a vander Waals gap between strongly bonded layers may be fast, but it canalso be accompanied by unwanted species from a liquid electrolyte. Onthe other hand, strong 3D bonding within a close-packed oxygen array, asoccurs in the spinel framework [Mn₂]O₄, offers too small a free volumefor the guest Li⁺ ions to have a high mobility at room temperature,which limits I_(max). Although this constraint in volume of theinterstitial space makes the spinel structure selective for insertion ofLi⁺ ions, it reduces the Li⁺-ion mobility and hence Li⁺-ion conductivityσ_(Li). The oxospinels have a sufficiently high σ_(Li), to be usedcommercially in low-power cells (Thackeray et al., 1983) but would notbe acceptable for the high power cells of the insertion.

The present invention overcomes these drawbacks by providing cathodematerials containing larger tetrahedral polyanions which form 3Dframework host structures with octahedral-site transition-metal oxidantcations. In the cathode materials of the invention having the NASICONstructure, the transition-metal ions are separated by the polyanions, sothe electronic conductivity is polaronic rather than metallic.Nevertheless, the gain in σ_(Li) more than offsets the loss inelectronic conductivity.

Variation of the energy of a given cation redox couple from one compoundto another depends on two factors: (a) the magnitude of the crystallineelectric field at the cation, which may be calculated for a purely ionicmodel by a Madelung summation of the Coulomb fields from the other ionspresent, and (b) the covalent contribution to the bonding, which may bemodulated by the strength of the covalent bonding at a nearest-neighborcounter cation. The stronger is the negative Madelung potential at acation, the higher is a given redox energy of a cation. Similarly thestronger is the covalent bonding of the electrons at a transition-metalcation, the higher is a given redox energy of that cation. The lower theredox energy of the cation host transition-metal ion, the larger isV_(oc).

The redox couples of interest for a cathode are associated withantibonding states of d-orbital parentage at transition-metal cations Mor 4f-orbital parentage at rare-earth cations Ln in an oxide. Thestronger is the cation-anion covalent mixing, the higher is the energyof a given LUMO/HOMO redox couple. Modulation of the strength of thecation-anion covalence at a given M or Li cation by nearest-neighborcations that compete for the same anion valence electrons is known asthe inductive effect. Changes of structure alter primarily the Madelungenergy as is illustrated by raising of the redox energy within a spinel[M₂]O₄ framework by about 1 eV on transfer of Li⁺ ions from tetrahedralto octahedral interstitial sites. Changing the counter cation, but notthe structure, alters primarily the inductive effect, as is illustratedby a lowering of the Fe³⁺/Fe²⁺ redox energy by 0.6 eV on changing(MoO₄)²⁻ or (WO₄)²⁻ to (SO₄)²⁻ polyanions in isostructural Fe₂(XO₄)₃compounds. Raising the energy of a given redox couple in a cathodelowers the voltage obtained from cells utilizing a common anode.Conversely, raising the redox energy of an anode raises the cell voltagewith respect to a common cathode.

The invention provides new cathode materials containing oxidepolyanions, including the oxide polyanion (PO₄)³⁻ as at least oneconstituent, for use in secondary (rechargeable) batteries. For example,the cathode materials of the present invention may have the generalformula LiM(PO₄) with the ordered olivine structure, or the more openrhombohedral NASICON framework structure. The cathode materials of thepresent invention have the general formula LiM(PO₄) for the orderedolivine structure, or Y_(x)M₂(PO₄)_(y)(XO₄)_(3-y) for the rhombohedralNASICON framework structure.

The olivine structure of Mg₂SiO₄ consists of a slightly distorted arrayof oxygen atoms with Mg²⁺ ions occupying half the octahedral sites intwo different ways. In alternate basal planes, they form zigzag chainsof corner-shared octahedra running along the c-axis and in the otherbasal planes they form linear chains of edge-shared octahedra runningalso along the c-axis.

In the ordered LiMPO₄ olivine structures of the invention, the M atomsoccupy the zigzag chains of octahedra and the Li atoms occupy the linearchains of the alternate planes of octahedral sites. In this embodimentof the present invention, M is preferably Mn, Fe, Co, Ni or combinationsthereof. Removal of all of the lithium atoms leaves the layeredFePO₄-type structure, which has the same Pbnm orthorhombic space group.These phases may be prepared from either end, e.g., LiFePO₄ (triphylite)or FePO₄ (heterosite), by reversible extraction or insertion of lithium.

FIG. 2A, FIG. 2B and FIG. 2C show discharge/charge curves vs. lithium at0.05 mA·cm⁻² (0.95 and 1.13 mA·g⁻¹, respectively) for Li_(1−x)FePO₄,Li_(1−x)Fe_(0.5)Mn_(0.5)PO₄ and LiFePO₄, respectively, where 0≦x≦0.5. Aplateau at 3.4 V corresponds to the Fe³⁺/Fe²⁺ redox couple and a plateauat 4.1 V corresponds to the Mn³⁺/Mn²⁺ couple. With LiClO₄ in PC and DMEas the electrolyte, it is only possible to charge up a cathode to 4.3 Vvs. a lithium anode, so it was not possible to extract lithium fromLiMnPO₄, LiCoPO₄ and LiNiPO₄ with this electrolyte. However, in thepresence of iron, the Mn³⁺/Mn²⁺ couple becomes accessible. Theinaccessibility is due to the stability of the Mn³⁺/Mn²⁺, Co³⁺/Co²⁺ andNi³⁺/Ni²⁺ couples in the presence of the polyanion (PO₄)³⁻. Therelatively strong covalence of the PO₄ tetrahedron of the compounds ofthe present invention stabilizes the redox couples at the octahedralsites to give the high V_(oc)'s that are observed.

Insertion of lithium into FePO₄ was reversible over the several cyclesstudied. FIG. 3 shows discharge/charge curves of FePO₄/LiClO₄+PC+DME/Licoin cell at 185 mA·g⁻¹ for FePO₄ prepared by chemical extraction of Li(delithiation) from LiFePO₄. The Li_(x)FePO₄ material of the presentinvention represents a cathode of good capacity and containsinexpensive, environmentally benign elements. While a nearlyclose-packed-hexagonal oxide-ion array apparently provides a relativelysmall free volume for Li⁺-ion motion, which would seem to support onlyrelatively small current densities at room temperature, increasing thecurrent density does not lower the closed-circuit voltage V. Rather, itdecreases, reversibly, the cell capacity. Capacity is easily restored byreducing the current.

As illustrated schematically in FIG. 4, lithium insertion proceeds fromthe surface of the particle moving inwards behind a two-phase interface.In the system shown, it is a Li_(x)FePO₄/Li_(1−x)FePO₄ interface. As thelithiation proceeds, the surface area of the interface shrinks. For aconstant rate of lithium transport per unit area across the interface, acritical surface area is reached where the rate of total lithiumtransported across the interface is no longer able to sustain thecurrent. At this point, cell performance becomes diffusion-limited. Thehigher the current, the greater is the total critical interface areaand, hence, the smaller the concentration x of inserted lithium beforethe cell performance becomes diffusion-limited. On extraction oflithium, the parent phase at the core of the particle grows back towardsthe particle surface. Thus, the parent phase is retained on repeatedcycling and the loss in capacity is reversible on lowering the currentdensity delivered by the cell. Therefore, this loss of capacity does notappear to be due to a breaking of the electrical contact betweenparticles as a result of volume changes, a process that is normallyirreversible. Moreover, the problem of decrease of capacity due to theparticles' breaking of electrical contact may be overcome by reducingthe particle size of the cathode material to the nanometer scale.

The invention further provides new cathode materials exhibiting arhombohedral NASICON framework. NASICON, as used herein, is an acronymfor the framework host of a sodium superionic conductorNa_(1+3x)Zr₂(P_(1-x)Si_(x)O₄)₃. The compound Fe₂(SO₄)₃ has two forms, arhombohedral NASICON structure and a related monoclinic form (Goodenoughet al. 1976; Long et al. 1979). Each structure contains units of twoFeO₆ octahedra bridged by three corner-sharing SO₄ tetrahedra. Theseunits form 3D frameworks by the bridging SO₄ tetrahedra of one unitsharing corners with FeO₆ octahedra of neighboring Fe₂(SO₄)₃ elementarybuilding blocks so that each tetrahedron shares corners with onlyoctahedra and each octahedron with only tetrahedra. In the rhombohedralform, the building blocks are aligned parallel, while they are alignednearly perpendicular to one another in the monoclinic phase. Thecollapsed monoclinic form has a smaller free volume for Li⁺-ion motionwhich is why the rhombohedral form is preferred. In these structures,the FeO₆ octahedra do not make direct contact, so electron transfer froman Fe²⁺ to an Fe³⁺ ion is polaronic and therefore activated.

Li_(x)Fe₂(SO₄)₃ has been reported to be a candidate material for thecathode of a Li⁺-ion rechargeable battery with a V_(oc)=3.6 V vs. alithium anode (Manthiram and Goodenough 1989). While the sulfates wouldseem to provide the desired larger free volume for Li, batteries usingsulfates in the cathode material tend to exhibit phase-transitionproblems, lowering the electronic conductivity. The reversible lithiuminsertion into both rhombohedral and monoclinic Fe₂(SO₄)₃ gives a flatclosed-circuit voltage vs. a lithium anode of 3.6 V (Manthiram andGoodenough 1989; Okada et al. 1994; Nanjundaswamy et al. 1996). Neitherparent phase has any significant solid solution with the orthorhombiclithiated phase Li₂Fe₂(SO₄)₃, which is derived from the rhombohedralform of Fe₂(SO₄)₃ by a displacive transition that leaves the frameworkintact. Powder X-ray diffraction verifies that lithiation occurs via atwo-phase process (Nanjundaswamy et al. 1996). Increasing the currentdensity does not change significantly the closed-circuit voltage V, butit does reduce reversibly the capacity. The reduction in capacity for agiven current density is greater for the motion of the lithiatedinterface. The interstitial space of the framework allows fast Li⁺-ionmotion, but the movement of lithium across the orthorhombic/monoclinicinterface is slower than that across the orthorhombic/rhombohedralinterface, which makes the reversible loss of capacity with increasingcurrent density greater for the monoclinic than for the rhombohedralparent phase.

The cathode materials of the invention avoid the phase transition ofknown sulfate cathode materials by incorporating one or more phosphateions as at least one of the constituents of the cathode material. Therhombohedral R3c (NASICON) and monoclinic P2₁/n framework structures ofLi₃Fe₂(PO₄)₃ are similar to those for the sulfates described above, asillustrated in FIG. 5A and FIG. 5B.

A further embodiment of the invention is a rhombohedral NASICON cathodematerial having the formula A_(3-x)V₂ (PO₄)₃, where A may be Li, Na or acombination thereof. Rhombohedral A_(3-x)V₂(PO₄)₃ reversiblyintercalates 1.5 Li per formula unit for a discharge capacity of 100mAh·g⁻¹ with average closed-circuit voltage being 3.8 V vs. a lithiumanode (see FIG. 8). The voltage and capacity performances of therhombohedral A_(3-x)V₂ (PO₄)₃ compounds of the invention are comparableto the high-voltage cathode materials LiMn₂O₄ (4.0 V), LiCoO₂ (4.0 V)and LiNiO₂ (4.0 V). The large, open three-dimensional framework of VO₆octahedra and PO₄ tetrahedra allows an easy diffusion of the lithiumions, making it attractive for high-power batteries. A further advantageof this material is that it includes a cheaper and less toxictransition-metal element (V) than the already developed systems usingCo, Ni, or Mn.

EXAMPLES Example 1 Ordered Olivine LiMPO₄ Compounds

The ordered-olivine compound LiFePO₄ was prepared from intimate mixturesof stoichiometric proportions of Li₂CO₃ or LiOH.H₂O, Fe(CH₃CO₂)₂ andNH₄H₂PO₄.H₂O; the mixtures were calcined at 300-350° C. to eliminateNH₃, H₂O, and CO₂ and then heated in Ar at about 800° C. for 24 hours toobtain LiFePO₄. Similar solid-state reactions were used to prepareLiMnPO₄, LiFe_(1-x)Mn_(x)PO₄, LiCoPO₄ and LiNiPO₄. FePO₄ was obtainedfrom LiFePO₄ by chemical extraction of Li from LiFePO₄. Charge/dischargecurves for Li_(1−x)FePO₄ and discharge/charge cycles for Li_(x)FePO₄gave similar results with a voltage of almost 3.5 V vs. lithium for acapacity of 0.6 Li/formula unit at a current density of 0.05 mA·cm²⁻(See FIG. 2A and FIG. 2C). The electrolyte used had a window restrictingvoltages to V<4.3 V. Li extraction was not possible from LiMnPO₄,LiCoPO₄, and LiNiPO₄ with the electrolyte used because these require avoltage V>4.3 V to initiate extraction. However, Li extraction fromLiFe_(1-x)Mn_(x)PO₄ was performed with 0≦x≦0.5, and the Mn³⁺/Mn²⁺ coupleproduced a voltage plateau at 4.0 V vs. lithium.

Arsenate may be substituted for at least some of the phosphate inLiFePO₄. The iron in LiFePO₄ may be supplemented up to 10% by othermetals such as those filled with two electron shells, manganese ortitanium, for example.

This iron-based material is non-toxic, inexpensive, non-hygroscopic,easy to prepare and has good electronic conductivity compared toexisting high voltage cathode materials. There is a decrease in thevolume of the unit cell with the extraction of lithium from LiFePO₄. Athigher current densities, there is a decrease of capacity on repeatedcycling associated with movement of a two-phase interface, a featurecharacteristic of cathodes that traverse a two-phase compositionaldomain in a discharge cycle. The volume change across the two-phaseinterface causes the particles to crack and lose electrical contact withthe current collector. This problem can be overcome by reducing theparticle size of the cathode material to the nanometer scale.

Example 2 Rhombohedral NASICON Li_(x)M₂(PO₄)O₃ Structures

The inventors compared redox energies in isostructural sulfates withphosphates to obtain the magnitude of the change due to the differentinductive effects of sulfur and phosphorus. RhombohedralLi_(1−x)Ti₂(PO₄)₃ has been shown to exhibit a flat open-circuit voltageV_(oc)=2.5 V vs. lithium, which is roughly 0.8 V below the Ti⁴⁺/Ti³⁺level found for FeTi(SO₄)₃. The flat voltage V(x) is indicative of atwo-phase process. A co-existence of rhombohedral and orthorhombicphases was found for x=0.5 (Delmas and Nadiri 1988; Wang and Hwu 1992).Li_(2+x)FeTi(PO₄)₃ of the present invention remains single phase ondischarge.

All three phosphates Li₃M₂(PO₄)₃, where M=Fe, Fe/V, or V, have themonoclinic Fe₂(SO₄)₃ structure if prepared by solid-state reaction. Theinventors have found that these compounds exhibit a rhombohedralstructure when prepared by ion exchange in LiNO₃ at 300° C. from thesodium analog NaFe₂(PO₄)₃. The discharge/charge curve of FIG. 6A forlithium insertion into rhombohedral Li_(3+x)Fe₂(PO₄)₃ exhibits anaverage V_(oc) of 2.8 V. This is surprisingly different from the curvesfor the monoclinic form (See FIG. 6B). The inventors have found that upto two lithiums per formula unit can be inserted into Li₃Fe₂(PO₄)₃,leading to Li₅Fe₂(PO₄)₃. The Li⁺-ion distribution in the interstitialspace of Li_(3+x)Fe₂(PO₄)₃, where 0<x<2, appears to vary continuouslywith x with a high degree of disorder. FIG. 7A shows a reversiblecapacity loss on increasing the current density from 0.05 to 0.5mA·cm⁻². A reversible discharge capacity of 95 mAh·g⁻¹ is still observedfor rhombohedral Li_(3+x)Fe₂(PO₄)₃ at a current density of 20 mA·g⁻¹.This is much reduced compared to what is encountered with the monoclinicsystem (See FIG. 7B). With a current density of 23 mA·g⁻¹ (or 1mA·cm⁻²), the initial capacity of 95 mAh·g⁻¹ was maintained in a coincell up to the 40^(th) cycle.

Another cathode material of the present invention, Li₂FeTi(PO₄)₃, havingthe NASICON framework was prepared by solid-state reaction. Thismaterial has a voltage ranging from 3.0 to 2.5 V.

Rhombohedral TiNb(PO₄)₃ can be prepared by solid-state reaction at about1200° C. Up to three Li atoms per formula unit can be inserted, whichallows access to the Nb⁴⁺/Nb³⁺ couple at 1.8 V vs. lithium for x>2 inLi_(x)TiNb(PO₄)₃. Two steps are perhaps discernible in the compositionalrange 0<x<2; one in the range of 0<x<1 corresponds to the Ti⁴⁺/Ti³⁺couple in the voltage range 2.5 V<V<2.7 V and the other for 1<x<2 to theNb⁵⁺/Nb⁴⁺ couple in the range 2.2 V<V<2.5 V. It appears that these redoxenergies overlap. This assignment is based on the fact that theTi⁴⁺/Ti³⁺ couple in Li_(1+x)Ti₂(PO₄)₃ gives a flat plateau at 2.5 V dueto the presence of two phases, rhombohedral LiTi₂(PO₄)₃ and orthorhombicLi₃Ti₂(PO₄)₃. The presence of Nb in the structure suppresses theformation of the second phase in the range 0<x<2.

Rhombohedral LiFeNb(PO₄)₃ and Li₂FeTi(PO₄)₃ can be prepared by ionexchange with molten LiNO₃ at about 300° C. from NaFeNb(PO₄)₃ andNa₂FeTi(PO₄)₃, respectively. Two Li atoms per formula unit can beinserted reversibly into Li_(2+x)FeTi(PO₄)₃ with a little loss ofcapacity at 0.5 mA·cm⁻². Insertion of the first Liatom in the range 2.7V<V<3.0 V corresponds to the Fe³⁺/Fe²⁺ redox couple and of the second Liatom in the range of 2.5 V<V<2.7 V to an overlapping Ti⁴⁺/Ti³⁺ redoxcouple. The insertion of lithium into Li_(1+x)FeNb(PO₄)₃ gives a V vs. xcurve that further verifies the location of the relative positions ofthe Fe³⁺/Fe²⁺, Nb⁵⁺/Nb⁴⁺ redox energies in phosphates withNASICON-related structures. It is possible to insert three lithium atomsinto the structure, and there are three distinct plateaus correspondingto Fe³⁺/Fe²⁺ at 2.8 V, Nb⁵⁺/Nb⁴⁺ at 2.2 V, and Nb⁴⁺/Nb⁵⁺ at 1.7 V vs.lithium in the discharge curve.

The rhombohedral A_(3-x)V₂(PO₄)₃ compounds of the invention can beprepared by ionic exchange from the monoclinic sodium analogNa₃V₂(PO₄)₃. The inventors were also able to prepare the rhombohedralLi₂NaV₂(PO₄)₃ with the NASICON framework by a direct solid-statereaction (FIG. 9). The discharge/charge curves at 0.05 mA·cm⁻² (0.95mA·g⁻¹) for the rhombohedral Li_(x)NaV₂(PO₄)₃ are shown in FIG. 8.

The rhombohedral LiFe₂(SO₄)₂(PO₄) may be prepared by obtaining anaqueous solution comprising FeCl₃, (NH₄)₂SO₄, and LiH₂PO₄, stirring thesolution and evaporating it to dryness, and heating the resulting drymaterial to about 500° C. Discharge/charge curves vs. lithium at 0.1mA·cm⁻² for rhombohedral Li_(1+x)Fe₂(PO₄)(SO₄)₂, where 0<x<3, are shownin FIG. 10.

All of the compositions and/or methods disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations may be applied tothe compositions and/or methods and in the steps or in the sequence ofsteps of the method described herein without departing from the concept,spirit and scope of the invention. More specifically, it will beapparent that certain agents which are both chemically and structurallyrelated may be substituted for the agents described herein to achievesimilar results. All such substitutions and modifications apparent tothose skilled in the art are deemed to be within the spirit, scope andconcept of the invention as defined by the appended claims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference:

-   Delmas, C., and A. Nadiri, Mater. Res. Bull., 23, 63 (1988);-   Goodenough, J. B., H. Y. P. Hong and J. A. Kafalas, Mater. Res.    Bull. 11, 203, (1976);-   Guyomard, D. and J. M. Tarascon, J. Electrochem. Soc., 139, 937    (1992);-   Long, G. J., G. Longworth, P. Battle, A. K. Cheetham, R. V.    Thundathil and D. Beveridge, Inorg. Chem., 18, 624 (1979);-   Manthiram, A., and J. B. Goodenough, J. Power Sources, 26, 403    (1989);-   Masquelier, C., M. Tabuchi, K. Ado, R. Kanno, Y. Kobayashi, Y. Maki,    O, Nakamura and J. B. Goodenough, J. Solid State Chem., 123, 255    (1996);-   Mizushima, K., P. C. Jones, P. J. Wiseman and J. B. Goodenough,    Mater. Res. Bull., 15, 783 (1980);-   Nanjundaswamy, K. S., et al., “Synthesis, redox potential evaluation    and electrochemical characteristics of NASICON-related 3D framework    compounds,” Solid State Ionics, 92 (1996) 1-10;-   Nishi, Y., H. Azuma and A. Omaru, U.S. Pat. No. 4,959,281, Sep. 25,    1990;-   Okada, S., K. S, Nanjundaswamy, A. Manthiram and J. B. Goodenough,    Proc. 36th Power Sources Conf., Cherry Hill at New Jersey (Jun. 6-9,    1994);-   Schöllhorn, R. and A. Payer, Agnew. Chem. (Int. Ed. Engl.), 24, 67    (1985);-   Sinha, S, and D. W. Murphy, Solid State Ionics, 20, 81 (1986);-   Thackeray, M. M. W. I. F. David, J. B. Goodenough and P. Groves,    Mater. Res. Bull., 20, 1137 (1983);-   Thackeray, M. M., P. J. Johnson, L. A. de Piciotto, P. G. Bruce    and J. B. Goodenough, Mater. Res. Bull., 19, 179 (1984);-   Thackeray, M. M., W. I. F. David, P. G. Bruce and J. B. Goodenough,    Mater. Res. Bull. 18, 461 (1983); and Wang, S., and S. J. Hwu, Chem.    of Mater. 4, 589 (1992).

1. A synthesized cathode material for a rechargeable electrochemicalcell comprising one or more compounds, at least one of the compoundswith an olivine structure comprising the general formula LiMPO₄, whereinM comprises Mn and up to 10% of one or more other metals.
 2. The cathodematerial according to claim 1, wherein one of the one or more othermetals comprises Fe.
 3. The cathode material according to claim 1,wherein the cathode material has at least one discharge plateau at about4.1 Volts versus lithium, corresponding to a Mn³⁺/Mn²⁺ couple.
 4. Thecathode material according to claim 1, wherein the compound with anolivine structure is contained in particles with a particle size on thenanometer scale.
 5. The cathode material according to claim 1, whereinthe compound with an olivine structure has a plurality of planes definedby zig-zag chains and linear chains, wherein lithium atoms occupy thelinear chains of alternate planes of octahedral sites.
 6. The cathodematerial according to claim 5, wherein M atoms occupy the zigzag chainsof octahedral sites of alternate planes of octahedral sites.
 7. Thecathode material according to claim 1, wherein one of the one or moreother metals comprises Ti.
 8. The cathode material according to claim 1,wherein one of the one or more other metals comprises a metal filledwith two electron shells.
 9. The cathode material according to claim 1,wherein the up to 10% one or more other metals comprises at least twoadditional metals.
 10. The cathode material according to claim 1,wherein one of the one or more other metals comprises Co.
 11. Arechargeable electrochemical cell comprising a cathode comprising one ormore compounds, at least one of the compounds with an olivine structurecomprising the general formula LiMPO₄, wherein M comprises Mn and up to10% of one or more other metals.
 12. The rechargeable electrochemicalcell according to claim 11, wherein the compound with an olivinestructure is contained in particles with a particle size on thenanometer scale.
 13. The rechargeable electrochemical cell according toclaim 11, wherein the cathode material has at least one dischargeplateau at about 4.1 Volts versus lithium, corresponding to a Mn³⁺/Mn²⁺couple.
 14. The rechargeable electrochemical cell according to claim 11,wherein one of the one or more other metals comprises Fe.
 15. Therechargeable electrochemical cell according to claim 11, wherein one ofthe one or more other metals comprises Ti.
 16. The rechargeableelectrochemical cell according to claim 11, wherein one of the one ormore other metals comprises Co.
 17. The rechargeable electrochemicalcell according to claim 11, wherein one of the one or more other metalscomprises a metal filled with two electron shells.
 18. The rechargeableelectrochemical cell according to claim 11, wherein the up to 10% one ormore other metals comprises at least two additional metals.
 19. Therechargeable electrochemical cell according to claim 11, wherein thecompound with an olivine structure has a plurality of planes defined byzig-zag chains and linear chains, wherein lithium atoms occupy thelinear chains of alternate planes of octahedral sites.
 20. Therechargeable electrochemical cell according to claim 19, wherein M atomsoccupy the zigzag chains of octahedral sites of alternate planes ofoctahedral sites.
 21. The cathode material according to claim 1, whereinM comprises Mn and up to 10% of one or more other transition metals. 22.The cathode material according to claim 13, wherein M comprises Mn andup to 10% of one or more other transition metals.